b. The influence of diffraction on the leeward side of groins which 

 has an effect in the immediate vicinity of the shoreline. 



c. The effect of changing, wave direction caused by refraction changes 

 the magnitude of longshore transport rate and the boundary conditions. 



d. The nonlinearity in thp transport equation is of minor importance 

 for small angles of incidence (for a> 45° , the coastline becomes 

 unstable as previously mentioned) . 



The two- line theory has been verified experimentally (Hulsbergen, 

 Van Bochove, and Bakker, 1976), and shows a trend identical to the ex- 

 perimental results. There are some differences at a small scale due to 

 secondary currents, breaking wave type, changes of wave height due to 

 small changes in morphology, etc. These, however ,.»'$re short-term 

 rather than long-term evolution phenomena. 



IV. THE EFFECT OF WAVE DIFFRACTION 



The effect of wave diffraction was subsequently taken into account 

 by Bakker (1970). Initially, this was done for the one-line theory of 

 Pelnard-Considere and later for Bakker 's two-line theory. 



Pelnard-Considere ' s equations. 



Q = Q^(x) - q(x) -g , q(x) 



(42) 



and 



9y ^ qOO 3Q(x) (43) 



3t D dx 



Still apply. Q and q are now functions of x , since both the 

 incident wave height and angle of approach vary along the shore with 

 X , because of wave diffraction. 



Inserting the expression for Q in the continuity equation, yields: 



3t D I ' 2 3x 3x I D dx ^ ' 



39 



